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Problems With NASA
Feb. 7th, 2006 11:31 amThe current incarnation of NASA - the same one we've had for about 30 years - is exactly the wrong type of agency to be in charge of our nation's space exploration and development work. It ruins worthy projects by requiring gold-plating which serves no practical purpose, often at the expense of design features which do.
An example of this is how they have altered the plans for the new Crew Launch Vehicle and Crew Exploration Vehicle. With each iteration, what had been worthwhile designs are degraded, as NASA insists on changes which increase cost and reduce value. For example, instead of using proven space-storable propellant combinations such as mixed hydrazine and nitrogen tetroxide in the CEV they are going with storing cryogenic fuel and oxidizer in space long term. The claim is that the cryogenic propellants will increase performance (as judged by the specific impulse available from the propellants). The added mass for the amount of insulation needed and the larger tanks due to the lower density fuel mean a heavier vehicle with lower performance. Here's a comparison for typical values, assuming an engine designed for vacuum use:
Vacuum Combined Fuel Oxidizer/
Oxidizer Fuel Isp Density Density Fuel Ratio
LOX Methane 388 0.801 0.423 3.5:1
LOX Subcooled Propane 385 1.1 0.782 3.3:1
LOX LH2 455 0.358 0.070 6.0:1
N2O4 Hydrazine/UDMH 370 1.2 0.895 2.15:1
The density of liquid oxygen is 1.14, which is good, but nitrogen tetroxide is better at 1.45. The mixture ratio in the hydrazine/unsymmetrical dimethyl hydrazine fuel is 50% of each; the commercial name is Aerozine-50 but it is often just called "fifty-fifty."
Denser propellants mean smaller tanks for the same total impulse, which means the tanks are lighter. Insulation is needed for all these, to keep them from vaporizing or freezing. Less dense propellants mean larger, heavier tanks which have larger surface areas and therefore need even more insulation. Then increase the thickness for cryogenics. (Note that in space there will naturally be no convection exchange, and very little conduction exchange, that through the plumbing. The insulation is to prevent radiative heat exchange, either inward from the Sun, or outward into space.)
There's no significant improvement in specific impulse (these numbers taken from high-performance existing engines) until you reach the LOX/LH2 combination, and that is very low-density. Hydrogen makes things even worse by being a deep cryogenic, needing not only extreme amounts of insulation, but perhaps even refrigeration equipment.
The N2O4/Mixed Hydrazines combination has the further advantage of being hypergolic, so no ignition system is needed. This simplifies the engine design and improves reliability. Yes, this combination has been used for something like fifty years. However, the laws of physics haven't changed and chemicals still react the same. Engine improvements generally are applicable to all common propellant combinations, so increases in performance are available across the board.
Therefore, for a space vehicle intended for long missions, using non-cryogenic, reasonably dense propellants will provide better total impulse for the same total vehicle mass.
Any competent engineer knows you have to balance various factors, including budget, when designing something. For a spacecraft this means not going for ultimate performance or flashy high tech innovations, it means sticking with what you know will do the job effectively. NASA has forgotten this and refuses to relearn it, in spite of _Challenger_ and _Columbia_.
An example of this is how they have altered the plans for the new Crew Launch Vehicle and Crew Exploration Vehicle. With each iteration, what had been worthwhile designs are degraded, as NASA insists on changes which increase cost and reduce value. For example, instead of using proven space-storable propellant combinations such as mixed hydrazine and nitrogen tetroxide in the CEV they are going with storing cryogenic fuel and oxidizer in space long term. The claim is that the cryogenic propellants will increase performance (as judged by the specific impulse available from the propellants). The added mass for the amount of insulation needed and the larger tanks due to the lower density fuel mean a heavier vehicle with lower performance. Here's a comparison for typical values, assuming an engine designed for vacuum use:
Vacuum Combined Fuel Oxidizer/
Oxidizer Fuel Isp Density Density Fuel Ratio
LOX Methane 388 0.801 0.423 3.5:1
LOX Subcooled Propane 385 1.1 0.782 3.3:1
LOX LH2 455 0.358 0.070 6.0:1
N2O4 Hydrazine/UDMH 370 1.2 0.895 2.15:1
The density of liquid oxygen is 1.14, which is good, but nitrogen tetroxide is better at 1.45. The mixture ratio in the hydrazine/unsymmetrical dimethyl hydrazine fuel is 50% of each; the commercial name is Aerozine-50 but it is often just called "fifty-fifty."
Denser propellants mean smaller tanks for the same total impulse, which means the tanks are lighter. Insulation is needed for all these, to keep them from vaporizing or freezing. Less dense propellants mean larger, heavier tanks which have larger surface areas and therefore need even more insulation. Then increase the thickness for cryogenics. (Note that in space there will naturally be no convection exchange, and very little conduction exchange, that through the plumbing. The insulation is to prevent radiative heat exchange, either inward from the Sun, or outward into space.)
There's no significant improvement in specific impulse (these numbers taken from high-performance existing engines) until you reach the LOX/LH2 combination, and that is very low-density. Hydrogen makes things even worse by being a deep cryogenic, needing not only extreme amounts of insulation, but perhaps even refrigeration equipment.
The N2O4/Mixed Hydrazines combination has the further advantage of being hypergolic, so no ignition system is needed. This simplifies the engine design and improves reliability. Yes, this combination has been used for something like fifty years. However, the laws of physics haven't changed and chemicals still react the same. Engine improvements generally are applicable to all common propellant combinations, so increases in performance are available across the board.
Therefore, for a space vehicle intended for long missions, using non-cryogenic, reasonably dense propellants will provide better total impulse for the same total vehicle mass.
Any competent engineer knows you have to balance various factors, including budget, when designing something. For a spacecraft this means not going for ultimate performance or flashy high tech innovations, it means sticking with what you know will do the job effectively. NASA has forgotten this and refuses to relearn it, in spite of _Challenger_ and _Columbia_.
